Begin by using comparisons between two towers of cubes, e.g. 5 an 8. Note the difference as being the number of cubes needed to be added to make the towers equal or subtracted to make the towers equal. When this concept is grasped very well move on to work on a numberline / hundred square. Forget, at this point, about equating this with subtraction as this will open up many avenues of confusion - you have taught subtraction as taking away from a set, then counting back on a numberline / 100square; wouldn't you be confused by someone now telling you you can subtract by counting up?

Youcan have fun introducing the concept by writing two numbers [say 5 and 8] on the board and initing the children to suggest what is meant by the difference. You'll get some fascinating suggestions, all quite legitimate!